Firebase Stream.asFuture（）在刷新小部件时复制数据

当我第一次加载小部件时，它会给我一个错误，但工作正常。当我转到另一个屏幕，然后返回，虽然它将复制来自未来的数据

@override
  Widget build(BuildContext context) {
   

    final userdata = context.watch<UserDataNotifier>();


    UserData data = Provider.of<UserData>(context);

    double h = MediaQuery.of(context).size.height;

    // print(data);
    return StreamBuilder(
      stream: Stream.fromFuture(data.getTheUserClasses),
      builder: (context, snapshot) {
        snapshot.data.toString();
        return Scaffold(
          backgroundColor: Color(0xff3DDC97),
          appBar: AppBar(
            backgroundColor: Color(0xff7211E0),
            title: userdata.user == null
                ? CircularProgressIndicator()
                : Text(userdata.user.firstName ?? ""),
          ),
          body: Container(
              // padding: EdgeInsets.symmetric(vertical: h / 8),
              padding: EdgeInsets.fromLTRB(0, h / 8, 0, 0),
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  SizedBox(
                      height: h * .5,
                      child: data.classList == null
                          ? Loading()
                          : data.classList.isEmpty
                              ? Loading()
                              : UserClassList(
                                  data: data.classList,
                                )),
                  RaisedButton(
                    child: Text("Add Class"),
                    onPressed: () {
                      Navigator.push(
                              context,
                              MaterialPageRoute(
                                  builder: (context) => PickFromAllClasses()))
                          .then((value) => value ? _refresh() : null);
                    },
                    shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(20)),
                    color: Colors.blue,
                  )
                ],
              )),
        );
      },
    );
  }

@覆盖
小部件构建（构建上下文）{
final userdata=context.watch（）；
UserData=Provider.of（上下文）；
double h=MediaQuery.of（context）.size.height；
//打印（数据）；
返回流生成器(
stream:stream.fromFuture（data.gettheUserClass），
生成器：（上下文，快照）{
snapshot.data.toString（）；
返回脚手架(
背景颜色：颜色（0xff3DDC97），
appBar:appBar(
背景颜色：颜色（0xff7211E0），
标题：userdata.user==null
？循环压缩机指示器（）
：Text（userdata.user.firstName？？），
),
主体：容器(
//填充：边缘组。对称（垂直：h/8），
填充：从LTRB（0，h/8，0，0）开始的边缘设置，
子：列(
mainAxisAlignment:mainAxisAlignment.center，
mainAxisSize:mainAxisSize.min，
儿童：[
大小盒子(
高度：h*.5，
子项：data.classList==null
？加载（）
：data.classList.isEmpty
？加载（）
：UserClassList(
数据：data.classList，
)),
升起的按钮(
子项：文本（“添加类”），
已按下：（）{
导航器。推(
上下文
材料路线(
生成器：（上下文）=>PickFromAllClasses（））
.然后（（值）=>值？\u refresh（）：空）；
},
形状：圆形矩形边框(
边界半径：边界半径。圆形（20）），
颜色：颜色，蓝色，
)
],
)),
);
},
);
}

下面是我获取数据的方法

    class UserData {
  String uid;
  String firstName;
  int rating;
  List<String> classes;
  List<ClassData> classList = List<ClassData>();

  UserData.fromMap(Map<String, dynamic> data) {
    firstName = data['firstname'] ?? "";
    rating = data['rating'] ?? "";
    classes = data['classes'].cast<String>() ?? "";
  }

 

  List<ClassData> get cs => classList;

  set cs(List<ClassData> s) {
    cs = s;
  }

  Future get getTheUserClasses async {
    for (String c in classes) {
      DocumentSnapshot classsnapshot =
          await Firestore.instance.collection("Classes").document(c).get();

      final data =
          ClassData.fromUserMap(classsnapshot.data, classsnapshot.documentID);

      if (data != null) {
        classList.add(data);
        // print(data.classdescription);
      }
    }

    cs = classList;
  }

  UserData({this.firstName, this.rating, this.classes});
}

类用户数据{
字符串uid；
字符串名；
国际评级；
列出课程；
List classList=List（）；
UserData.fromMap（地图数据）{
firstName=数据['firstName']？？“”；
评级=数据[“评级”]？“”；
classes=数据['classes'].cast（）？？“”；
}
List get cs=>classList；
设置cs（列表s）{
cs=s；
}
Future GetTheUserClass异步{
for（类中的字符串c）{
文档快照类快照=
等待Firestore.instance.collection（“Classes”）.document（c.get（）；
最终数据=
ClassData.fromUserMap（classsnapshot.data，classsnapshot.documentID）；
如果（数据！=null）{
添加（数据）；
//打印（data.classdescription）；
}
}
cs=类列表；
}
UserData（{this.firstName，this.rating，this.classes}）；
}

下面是加载此小部件时出现的错误

flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following NoSuchMethodError was thrown building YourClasses(dirty, dependencies: [MediaQuery,
flutter: _InheritedProviderScope<UserDataNotifier>, _InheritedProviderScope<UserData>], state:
flutter: _YourClassesState#4bf6d):
flutter: The getter 'getTheUserClasses' was called on null.
flutter: Receiver: null
flutter: Tried calling: getTheUserClasses
flutter:
flutter: The relevant error-causing widget was:
flutter:   YourClasses
flutter:   file:///Users/devintripp/Desktop/flutter_apps.no_sync/discoverytutors/lib/Screens/LoggedIn/TutorsView/tutors.dart:148:65
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
flutter: #1      _YourClassesState.build (package:disc_t/Screens/LoggedIn/Classes/yourclasses.dart:66:38)
flutter: #2      StatefulElement.build (package:flutter/src/widgets/framework.dart:4619:28)
flutter: #3      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4502:15)
flutter: #4      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4675:11)
flutter: #5      Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
flutter: #6      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4481:5)
flutter: #7      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4666:11)
flutter: #8      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4476:5)
flutter: ...     Normal element mounting (24 frames)
flutter: #32     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3446:14)
flutter: #33     MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5947:32)
flutter: ...     Normal element mounting (119 frames)
flutter: #152    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3446:14)
flutter: #153    Element.updateChild (package:flutter/src/widgets/framework.dart:3214:18)
flutter: #154    RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:5580:32)
flutter: #155    MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5957:17)
flutter: #156    Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
flutter: #157    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4527:16)
flutter: #158    StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4675:11)
flutter: #159    Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
flutter: #160    StatefulElement.update (package:flutter/src/widgets/framework.dart:4707:5)
flutter: #161    Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
flutter: #162    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4527:16)
flutter: #163    Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
flutter: #164    ProxyElement.update (package:flutter/src/widgets/framework.dart:4862:5)
flutter: #165    _InheritedNotifierElement.update (package:flutter/src/widgets/inherited_notifier.dart:181:11)
flutter: #166    Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
flutter: #167    SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5837:14)
flutter: #168    Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
flutter: #169    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4527:16)
flutter: #170    StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4675:11)
flutter: #171    Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
flutter: #172    StatefulElement.update (package:flutter/src/widgets/framework.dart:4707:5)
flutter: #173    Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
flutter: #174    SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5837:14)
flutter: #175    Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
flutter: #176    SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5837:14)
flutter: #177    Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
flutter: #178    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4527:16)
flutter: #179    Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
flutter: #180    StatelessElement.update (package:flutter/src/widgets/framework.dart:4583:5)
flutter: #181    Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
flutter: #182    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4527:16)
flutter: #183    StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4675:11)
flutter: #184    Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
flutter: #185    BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2627:33)
flutter: #186    WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:883:20)
flutter: #187    RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:284:5)
flutter: #188    SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1113:15)
flutter: #189    SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1052:9)
flutter: #190    SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:968:5)
flutter: #194    _invoke (dart:ui/hooks.dart:261:10)
flutter: #195    _drawFrame (dart:ui/hooks.dart:219:3)
flutter: (elided 3 frames from dart:async)

颤振：══╡ WIDGETS库捕获到异常╞═══════════════════════════════════════════════════════════
颤振：生成类时抛出以下NoSuchMethodError（脏的，依赖项：[MediaQuery，
颤振：_InheritedProviderScope，_InheritedProviderScope]，状态：
颤振：_yourclassestate#4bf6d）：
颤振：对null调用了getter'getTheUserClasses'。
颤振：接收器：空
颤振：尝试呼叫：GetTheUserClass
颤振：
颤振：导致错误的相关小部件是：
弗利特：你的课
颤振：file:///Users/devintripp/Desktop/flutter_apps.no_sync/discoverytutors/lib/Screens/LoggedIn/TutorsView/tutors.dart:148:65
颤振：
颤振：抛出异常时，这是堆栈：
颤振：#0 Object.noSuchMethod（省道：核心补片/对象补片。省道：53:5）
flatter:#1_yourclassestate.build（包：disc_t/Screens/LoggedIn/Classes/yourclass.dart:66:38）
flatter:#2 StatefulElement.build（包：flatter/src/widgets/framework.dart:4619:28）
flatter:#3 ComponentElement.performRebuild（包：flatter/src/widgets/framework.dart:4502:15）
flatter:#4 StatefulElement.performRebuild（包：flatter/src/widgets/framework.dart:4675:11）
flatter:#5 Element.rebuild（包：flatter/src/widgets/framework.dart:4218:5）
flatter:#6 ComponentElement._firstBuild（包：flatter/src/widgets/framework.dart:4481:5）
flatter:#7 StatefulElement._firstBuild（包：flatter/src/widgets/framework.dart:4666:11）
flatter:#8 ComponentElement.mount（包：flatter/src/widgets/framework.dart:4476:5）
颤振：。。。正常元件安装（24个机架）
flatter:#32 Element.inflateWidget（包：flatter/src/widgets/framework.dart:3446:14）
flatter:#33 MultiChildRenderObjectElement.mount（包：flatter/src/widgets/framework.dart:5947:32）
颤振：。。。正常元件安装（119机架）
flatter:#152 Element.inflateWidget（包：flatter/src/widgets/framework.dart:3446:14）
flatter:#153 Element.updateChild（包：flatter/src/widgets/framework.dart:3214:18）
flatter:#154 RenderObjectElement.updateChildren（包：flatter/src/widgets/framework.dart:5580:32）
flatter:#155 MultiChildRenderObjectElement.update（包：flatter/src/widgets/framework.dart:5957:17）
flatter:#156 Element.updateChild（包：flatter/src/widgets/framework.dart:3201:15）
flatter:#157 ComponentElement.performRebuild（包：flatter/src/widgets/framework.dart:4527:16）
flatter:#158 StatefulElement.performRebuild（包：flatter/src/widgets/framework.dart:4675:11）
flatter:#159 Element.rebuild（包：flatter/src/widgets/framework.dart:4218:5）
颤振：#160状态元素
Stream<List<YourModel>> getUserList() {
return Firestore.instance.collection('Classes')
    .snapshots()
    .map((snapShot) => snapShot.documents
    .map((document) => Yourmodel.fromDocument(document.data)
    .toList());
}